1. Field of the Invention
This invention relates to a display device used in electronic equipment and for displaying various character information and characters, in particular to a display device for displaying pictures by using organic electroluminescent (EL) elements in the display section.
2. Description of Related Art
As a flat-type display device based on the dot matrix display system, there is a display device in which luminescent elements such as organic electroluminescent (EL) elements are used. A luminescent element based on organic electroluminescent (EL) elements (hereinafter referred to as xe2x80x9corganic EL elementxe2x80x9d) is an element composed basically of a luminescent layer made of laminated fluorescent material in which electrodes of a small work function (electrodes for injection of electrons) are formed. Such organic EL elements are characterized by their ability to provide a very high luminance for a small voltage.
As shown in FIG. 6, a display device in which organic EL element is used 21 (hereinafter referred to simply as xe2x80x9cthe display devicexe2x80x9d) has organic layers (luminescent layers) 22a and 22b sandwiched between a plurality of common line electrodes 23 serving as negative poles and a plurality of segment line electrodes 24 serving as positive electrodes, and disposed between the transparent (glass) substrates 25 and 26. Any potential difference given between the common line electrodes 23 and the segment line electrodes 24 creates an electric field between the organic layers 22a and 22b, and electrons accelerated by this electric field creates electron-hole pairs. Alternatively, the application of an electric field may result in the disappearance of electron-hole pairs. The area of the part radiating at this time (hereinafter referred to as the xe2x80x9cluminescent surfacexe2x80x9d) will be the area of individual electrodes for injection of electrons sandwiching the organic layers 22a and 22b. 
Display devices developed in recent years adopted the dot matrix system for chronologically changing pictures and figures specifically formed for each character for fixed pictures. As figures specifically formed for each character are used for fixed pictures, curves can be used freely and freedom of display may be enhanced. A display device including both a display section based on a prior dot matrix system and a display section for displaying a figure formed into a specific character will be explained with reference to FIGS. 6 and 7.
The display device 21 includes a dot display portion 27 based on the dot matrix system, which is connected with a line electrode drive circuit 28 and a column electrode drive circuit 29 providing a potential difference to the organic layer 22a. Line electrode drive circuit 28 and column electrode drive circuit 29 are connected with the control section 30. Similarly, the display device 21 includes a figure display portion 31 for displaying a figure formed into a character, which is connected with a line electrode drive circuit 32 and a column electrode drive circuit 33 providing a potential difference to the organic layer 22b. The line electrode drive circuit 32 and column electrode drive circuit 33 are connected with the control section 34. Control sections 30 and 34 are designed to receive inputs of image signals and based on these image signals each of these control sections 30 and 34 control the line electrode drive circuits 28 and 32 and column electrode drive circuits 29 and 33 to change the potential differences given to the organic layers 22a and 22b of the dot display portion 27 and the figure display portion 31.
In case where the dot matrix system is used for displaying any picture, the reduction of each dot or radiating surface to a small area produces clearer pictures. On the other hand, where a figure formed into a specific character is to be displayed, the enlargement of the figure itself for display makes it easier to view. Thus, a figure formed into a character tends to grow far larger than a dot adopted for the dot matrix system. In other words, the area of electrodes into which electrons are injected to radiate a figure formed into a character on the organic layer 22b grows larger than the area of an electrode into which electrons are injected to radiate according to the dot matrix system. Therefore, even if the same potential difference is given to both of the organic layers 22a and 22b, the amount of electric current flowing into the latter organic layer 22b grows smaller. As a result, in comparison with the luminescent area of each dot according to the dot matrix system, the luminescent surface of the figure formed into a character grows darker.
For the above reason, in order to maintain the luminosity of the luminescent surface of the figure formed into a character at a level of luminosity similar to that of the luminescent surface of each dot displayed according to the dot matrix system, the potential difference between the common line electrodes and the segment line electrodes must be kept at a higher level than the potential difference applied to the organic layer 22a radiating according to the dot matrix system. Therefore, apart from the common line electrodes, the segment line electrodes, the line electrode drive circuit and the column electrode drive circuit for supplying electric current to the organic layer 22a radiating according to the dot matrix system, new common line electrodes, segment line electrodes, line electrode drive circuit and column electrode drive circuit must be created to provide for further potential difference, which resulted in additional components, further complication of the control section and added up the cost.
It is therefore an object of the present invention to provide a display device by which a figure formed into a character can radiate with an even luminosity as that of the luminescent surface according to the dot matrix system without increasing the number of components.
In order to solve the above problem, the display device according to one embodiment of the present invention includes the first luminescent section of a prescribed area and the second luminescent section forming a picture of an area larger than the first luminescent section, the first and the second luminescent sections being formed by a luminescent body radiating as electrons move and a pair of electrodes sandwiching this luminescent body, at least one of the electrodes determining the area of radiation, and at least the second luminescent section being formed by a plurality of divided electrodes.
In another embodiment, the display device according to the present invention includes a drive circuit for driving the segment line electrodes and common line electrodes for supplying electric current to the first luminescent section, and the drive circuit drives the second luminescent section.
In still another embodiment, the display device according to the present invention includes electrodes forming the second luminescent section divided so that each of them would take the same area as that of the first luminescent section.
In still another embodiment, the present invention provides a display device in which the electrodes of the second luminescent section are disposed in such a way as to form at least a character and the electrodes are divided by slits crossing with characters.
In another embodiment, the present invention provides a display device in which slits are formed by a line smaller than characters.
The presently preferred embodiment includes a first luminescent section of a prescribed area, i.e. a dot of the dot display portion 2, and a second luminescent section displaying a figure of an area larger than that of said first luminescent section, i.e. a figure display portion 3. As electrons move between the pair of electrodes sandwiching the first or second luminescent sections, the luminescent body radiates. The area of luminescence is determined by at least one of the electrodes. If the electrode of the second luminescent section is divided into a plurality of electrodes such that the area of each divided subsection of the second luminescent section becomes close to that of the first luminescent section, the luminosity of the second luminescent section can be brought to the same level of luminosity as that of the first luminescent section even if the same potential difference is applied.
By including a drive circuit for driving the segment line electrodes and the common line electrodes for supplying electric current to the first luminescent section and the use of this drive circuit to drive the second luminescent section, the display device according to the present invention can better apply a potential difference to the second luminescent section without increasing the number of components.
By utilizing a plurality of electrodes in the second luminescent section in order to provide luminescent subsections with an area substantially the same as the area of the first luminescent section, the display device according to the present invention can more accurately radiate the first and the second luminescent sections with an even luminosity by applying a similar potential difference to them.
By forming electrodes of the second luminescent section as slits which cross with the display characters, the display device according to the present invention enhances a user""s ability to distinguish lines constituting characters from slits even when the second luminescent section contains characters. Thus, visual recognizability is not impaired.
Further, the electrodes of the second luminescent section may be constructed with a width substantially smaller than the width of the lines constituting characters. This also reduces the likelihood of impaired visual recognizability.